The present invention relates to a wire cooling system for use in a wire drawing machine of the type wherein a wire is pulled through a die and thereby drawn and sized, i.e. the cross section of the wire is reduced by such drawing.
As in any mechanical deformation operation, the reduction of the cross section of a metal wire by drawing, i.e. by pulling a wire through a die by means of a winding capstan, generates heat which results in increasing the temperature of the wire being drawn and of the tools in contact therewith. Such increase of temperature changes the mechanical characteristics of the wire. Such change is all the more detrimental the higher is the carbon content of the wire, for example with a steel wire, and such change produces aging phenomena well known in the art. This particularly is true of dry-type drawing machines where the lubricant used to facilitate the drawing operation is a dry powdery product with no cooling function whatsoever.
To limit the heating of the wire and to maintain the temperature thereof within metallurgically acceptable limits, presently employed drawing machines are equipped with wire cooling systems which roughly can be divided into two categories, namely indirect cooling systems and direct cooling systems.
Indirect cooling is achieved by means of employing a cooling liquid to cool the tools that come into contact with the wire, i.e. the die and the capstan. Since the contact time of a given wire section with the die is extremely short, this cooling is very limited. By contrast, the capstan has in contact therewith a large number of turns of the wire, thereby increasing the surface of contact between the hot wire and the cooled capstan. Accordingly, this cooling is more effective, but still is limited by fundamental heat exchange principles on the one hand, and by the purely linear contact between the round wire and the cylindrical periphery of the capstan on the other hand.
Direct cooling can be employed alone or in combination with indirect cooling and generally is of two types, i.e. air-type cooling and liquid-type cooling.
In air-type cooling, air is blown around the capstan and creates an ascending air jacket to cool the wire turns accumulated on the periphery of the capstan. This process frequently is used together with internal cooling of the capstan by means of a liquid as described above. It should be noted that the smaller the diameter of the wire, the greater is the effectiveness of air-type cooling. Limitations of this arrangement however are obvious because air has a low specific heat. Further disadvantages of this arrangement are the resultant noise and ecologically detrimental stirring or blowing of soaps and residues that always are present in a wire drawing machine.
Several types of liquid-type cooling systems are known and involve spraying the wire during its passage between the delivery side of the die and the point of contact with the capstan. These systems have proved to be ineffective on high speed drawing machines because, in view of the limited space available, the time of contact of the cooling liquid with the wire ranges from only a few tenths of a second to a few hundredths of a second. Furthermore, upon the occurrence of a wire break, the cooling liquid may well pass through the die and mix with the drawing soap upstream of the die. It should be noted that this type of system usually requires substantially longer wire threading points and thus makes, the stringing of the machine more difficult and delicate.
In another known liquid-type cooling system, cooling liquid from an external source is sprayed directly onto the wire turns that have accumulated on the capstan, thereby subjecting the wire to cooling for a longer period of time. Such a system would seem generally to be more effective, but in practice has been found to be unacceptable due to dripping of the cooling liquid, centrifugation of the cooling liquid, and the resultant misty or moist atmosphere formed around the machine.
Since the production of a wire drawing machine is dependent upon its speed, wire manufacturers have been searching for increasingly fast machines. However, since the degree of heating of the wire also is directly dependent upon speed, one is presented with a conflict that is difficult to overcome.